In the formation of integrated circuit devices (ICs), a semiconducting substrate (e.g. silicon) is subjected to a series of chemical and thermal process steps to modify the electrical properties of certain areas of the substrate. Doping is the process of placing specific amounts of dopant atoms into the crystal lattice structure of the substrate or into a film deposited on the substrate.
In general, the electrical characteristics (e.g. conductivity, resistivity) of a defined region of a semiconductor structure are a function of the concentration and depth of the dopants in that region. Common dopants for silicon include phosphorous, boron and arsenic. In order to obtain semiconductor devices having predictable and reliable electrical characteristics, a doping process must be controlled to provide an optimal concentration and depth for the dopant atoms. In addition, the doping process must ensure that contaminants such as oxygen, carbon, H.sub.2 O and others are not introduced into the semiconductor devices during the doping process.
One technique for doping a semiconducting substrate is known as ion implantation. Ion implantation can be used to precisely control the number of implanted dopant atoms and their depth distribution profile within a semiconducting substrate. Following ion implantation a thermal annealing step is performed. The annealing step further drives the dopants into the crystal lattice.
With ion implantation the implanted ions are ionized atoms of the dopants. Ionization typically occurs in a process chamber that is fed by source vapors. The process chamber is maintained at a reduced atmospheric pressure or vacuum. Inside the process chamber is a filament that is heated to the point where electrons are created from the filament surface. The negatively charged electrons are attracted to an oppositely charged anode in the chamber. During the travel from the filament to the anode, the electrons collide with the dopant source molecules and create positively charged ions from the elements in the molecule. These ions are then accelerated to a high velocity and directed at a target semiconductor wafer. The ions possess enough energy to penetrate the surface of the wafer.
One problem associated with the use of ion implantation in the manufacture of integrated circuits is contaminants introduced into the crystal lattice structure of the substrate during the ion implantation process. These implanted contaminants are sometimes referred to as "knock ons". Typically, "knock ons" originate as contaminants such as adsorbed oxygen, carbon, carbon dioxide, H.sub.2 O atoms and native oxide layers (SiO.sub.2) present on the surface of the substrate to be implanted. During the ion implantation process these atoms are implanted into the crystal lattice structure of a substrate by impinging implant ions.
This is a problem because the presence of "knock ons" in the crystal lattice of a semiconducting substrate, adversely affects the completed semiconductor devices. As an example, the "knock ons" damage and contaminate the crystal lattice and may cause current leakage and undesirable electrical characteristics in the implanted substrate. This is a particular problem in the formation of high density integrated circuits. AS an example, during the arsenic implantation of silicon, it is very easy to introduce "knock ons" into the crystal lattice. Moreover, this problem is compounded by the increased use of new MEV implantation methods in the production of DRAMS.
In view of the foregoing, there is a need in the art for improved ion implantation methods. The present invention is directed to an improved method of ion implantation and particularly to an improved method for removing contaminants in-situ during ion implantation.
Accordingly, it is an object of the present invention to provide an improved ion implantation method in which contaminants are removed in-situ prior to implantation of the dopants. It is another object of the present invention to provide an improved ion implantation method which uses a hydrogen reduction to remove contaminants from the surface of a substrate to be implanted to prevent the implantation of "knock ons" into the crystal lattice structure of the substrate. It is a further object of the present invention to provide an improved ion implantation method that is suitable for large scale semiconductor manufacture.